Cyclization phenomena in the sol-gel polymerization of alpha,omega-bis(triethoxysilyl) alkanes and incorporation of the cyclic structures into network silsesquioxane polymers

Intramolecular cyclizations during acid-catalyzed sol-gel polymerizations o f alpha,omega-bis(triethoxysilyl)-alkanes substantially lengthen gel times for monomers with ethylene (1), propylene (2), and butylene (3) bridging gr oups. These cyclization reactions were found, using mass spectrometry and S i-29 NMR spectroscopy, to lead preferentially to monomeric and dimeric prod ucts based on six- and seven-membered disilsesquioxane rings. 1,2-Bis(triet hoxysilyl)ethane (1) reacts under acidic conditions to give a bicyclic dime r (5) that is composed of two annelated seven-membered rings. Under the sam e conditions, 1,3-bis(triethoxysilyl)propane (2), 1,4-bis(triethoxysilyl)bu tane (3), and Z-1,4-bis(triethoxysilyl)but-2-ene (10) undergo an intramolec ular condensation reaction to give the six- and seven-membered cyclic disil sesquioxanes 6, 7, and 11. Subsequently, these cyclic monomers slowly react to form the tricyclic dimers 8, 9, and 12. With NaOH as polymerization cat alyst, these cyclic silsesquioxanes readily reacted to afford gels that wer e shown by CP MAS Si-29 NMR and infrared spectroscopies to retain some cycl ic structures. Comparison of the porosity and microstructure of xerogels pr epared from the cyclic monomers 6 and 7 with those of gels prepared directl y from their acyclic precursors 2 and 3 indicates that the final pore struc ture of the xerogels is markedly dependent on the nature of the precursor. In addition, despite the fact that the monomeric cyclic disilsesquioxane sp ecies cannot be isolated from 1-3 under basic conditions due to their rapid rate of gelation, spectroscopic techniques also detected the presence of t he cyclic structures in the resulting polymeric gels.